Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, Polyethylene naphthalate (PEN), Polyetheretherketone (PEEK), transparent conductive polyester film, or for the very stretchy applications, silicone. Silicone substrates can support large strains of 10s to 100s of percent. High performance active inorganic electronics based on established technologies such as single crystal silicon or compound semiconductors can be integrated onto these substrates as islands of material, as these materials cannot support large strains. However, the metal interconnects required to transmit signals between these islands need to tolerate large strains without breaking.
Current approaches towards making flexible metal traces on a flexible substrate involve depositing metal conductors (lines) on pre-strained substrates using designed/controlled buckling patterns. When the pre-strained substrates are released, the deposited metal conductors buckle according to the controlled buckling patterns. When the substrates are subsequently stretched, the buckled sections are pulled into a partially flattened state, whereby electrical connections are maintained. Meandering metal lines are used to accommodate even larger strains.
There are multiple problems with the pre-strained, buckled metal line approach. Pre-tensioning the substrate is cumbersome, and not easily scalable. The meandering metal conductors cannot be patterned to achieve high signal density, as the meanders take up space, particularly for larger strain designs. Similarly, the conductivity is limited because the metal lines can't be wide. Also, the buckling design creates exposed out of plane structures, which is inherently fragile because thin film metal is protruding from the flexible substrate surface, and also does not allow for more complicated multilayer designs without large signal density tradeoffs. Meander designs can be stacked, but require thick (e.g., 300 μm) buffer layers to protect the protruding buckles, so achieving vertical interconnections between the layers would be very difficult and inherently low density due to the large buffer layer thickness.
What is needed is a reliable flexible metal interconnect structure for flexible electronics that has high density, accommodates large strains, and remains in-plane (i.e., does not buckle).